Although cyclic olefin polymers have high transparency and are useful as materials for optical elements, such as lenses, it is known that cyclic olefin polymers have high linear expansion coefficients.
It is known that a technique for cross-linking a cyclic olefin polymer produces a material having a low linear expansion coefficient. PTL 1 discloses the cross-link of a cross-linkable cyclic olefin polymer having the following formula (1).

In the formula (1), n1 denotes 1 or 2, n2 denotes an integer of 1 or more and 12 or less, R1, R2, and R3 independently denote a hydrogen atom or a hydrocarbon group having 1 or more and 10 or less carbon atoms, and R4 denotes a hydrogen atom or a methyl group.
However, the cyclic olefin polymer having the formula (1) disclosed in PTL 1 has a low glass transition temperature and a slightly high linear expansion coefficient.